Phase change record carries having crystalline nuclei and/or which produce crystallization structures when information is written thereon enabling that information to be more easily erased

ABSTRACT

Information storage system, device and record carriers for improved erasing of information written on a record carrier. A radiation beam is focused on a recording layer of a record carrier containing phase-change material. Crystalline information areas are written on initially amorphous portions of the recording layer by means of the radiation beam having its intensity modulated in accordance with write pulses. The information areas are erased (and changed back into amorphous portions) by means of the radiation beam having its intensity modulated in accordance with erase pulses. The time duration of each of the erase pulses (and, therefore, the intensity of the radiation beam being modulated in accordance therewith to heat the recording layer) and the cooling time of the recording layer thereafter is short enough that areas of the recording layer which are heated during erasing to a temperature above the crystallization temperature of the recording layer but lower than the melting point of the recording layer remain above the crystallization temperature for a period of time which is shorter than the crystallization time for the recording layer. The recording layer may contain a track and have more crystallization nuclei proximate to a centerline of the track than away from the centerline. In addition, the record carrier may be such that its reflection is larger when the recording layer has an amorphous structure than when it has a crystalline structure.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 08/107,532, filed Aug. 17,1993, now U.S. Pat. No. 5,383,172.

BACKGROUND OF THE INVENTION

The invention relates to record carriers. Such a record carrier has arecording layer in which information can be (a) written by changing anarea of the recording layer having an amorphous structure by means ofwrite pulses of a radiation beam into an information area having acrystalline structure and (b) erased by heating an information areaabove the melting point of the recording layer by means of erase pulsesof a radiation beam.

A system which is capable of writing and erasing information on such arecord carrier is known from an article entitled "Optical memory" by P.Chaudhari and C. B. Zarowin in the IBM Technical Disclosure Bulletin,vol. 16, no. 2, July 1973, pp. 568 and 569. In that system, acrystalline information area in an initially amorphous material iswritten by means of a pulsed and focused radiation beam. The informationarea is erased by rendering the material amorphous again by means of anerase pulse at the information area. The erase pulse has a shorter pulseduration and a larger amplitude than the write pulse. However, it hasbeen found that upon reading information which has been written aftererasure, a read signal is obtained which is not optimum and has asignal-to-noise ratio which is too low.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a a record carrierof the type described in the opening paragraph in which informationwritten after erasure can also be read satisfactorily.

The invention is based on the recognition that due to the erasure therecording layer does not entirely return to the amorphous state. It isfound that small non-amorphous (i.e., crystalline) areas are leftbehind. After a subsequent write action those small non-amorphous (i.e.,crystalline) areas, between information areas which have then beenwritten, impede reading of the record carrier.

The small non-amorphous (i.e., crystalline) areas arise as follows.During erasure, the recording layer is heated by a pulsed radiation beamfocused to a radiation spot which locally heats the recording layer witha bell-shaped temperature profile. An area around the center of theprofile is heated above the melting point. That central area isgenerally as large as, or slightly larger than, the crystallineinformation area. After the beam has been switched off, the central areacools down so rapidly that it solidifies in the amorphous structure.However, the radiation beam also heats an annular area of the recordinglayer, which is in the amorphous state, around the central area, to atemperature which is lower than the melting point but higher than thecrystallization temperature. (The crystallization temperature is thetemperature above which an area crystallizes within a period of timecommonly used for writing in optical information storage systems, i.e.,of the order of 1 μs.) The initially amorphous annular area willentirely or partly crystallize due to this heating. Outside the annulararea, the temperature of the recording layer remains below thecrystallization temperature so that this layer will not change itsstructure at that area. Those annular areas produce the above-mentionedread problems.

To crystallize an amorphous area, not only should the temperature ofthat area be raised above the crystallization temperature, but that areashould also remain at that temperature for a sufficiently long time toenable the atoms in the recording layer to order themselves from theamorphous structure into the crystalline structure. The time minimallyrequired for the recording layer to crystallize at a temperature justbelow the melting point will hereinafter be referred to as thecrystallization time. The crystallization time is dependent, inter aliaon the material of the recording layer. The time during which an area ofthe recording layer (e.g., the annular area) is above thecrystallization temperature is mainly determined by the pulse durationof the erase pulse and the cooling time, i.e., the time in which thearea cools down from a temperature just below the melting point to thecrystallization temperature.

According to an aspect of the invention, during erasing, an erase pulsehaving a short enough pulse duration is used in combination with a shortenough cooling time so that the annular area is heated above thecrystallization temperature for a period of time which is shorter thanthe crystallization time so that there will be no crystallization inthat area. As a result, that area remains amorphous so that aftererasing there are no completely or partly crystalline areas left whichmay impede reading of newly written information at a later stage.

In spite of the short pulse duration of the erase pulses,crystallization (i.e., creeping crystallization) may still occur undercertain circumstances after frequent erasing in areas between the tracksin which the information areas are written. During erasing, areas of therecording layer around the centerline of a track will be melted so thatcrystalline (i.e., creeping crystalline) areas which are possiblypresent are effectively erased. However, the areas (midway) between thetracks will at most be heated to a temperature below the melting pointand consequently will not be erased.

A record carrier, in accordance with the invention, in which creepingcrystallization is inhibited is characterized in that the recordinglayer has tracks in which the written areas are ordered and there aremore crystallization nuclei proximate to the centerlines of the tracksthan between the tracks. The low number of crystallization nucleibetween the tracks considerably slows down the crystallization in thatarea. However, a sufficient number of crystallization nuclei should bepresent around the centerlines of the tracks so that the crystallizationof the information areas to be written can be effected sufficientlyrapidly during writing.

A record carrier, in accordance with the invention, in which additionalcrystallization nuclei are provided in a simple manner is characterizedin that it has grooves in the middle of the tracks with sharp interfacesin the profile of the tracks, the sharp interfaces constituting thecrystallization nuclei. Those tracks can also be used for generating atracking error signal with which the position of the radiation spot onthe record layer can be controlled.

Another embodiment of a record carrier, in accordance with theinvention, is characterized in that the reflection of the record carrierwith the recording layer in the amorphous structure is larger than thatwith the recording layer in the crystalline structure. A higher powermust be absorbed in the recording layer for erasing a crystallineinformation area than for writing such an area because the recordinglayer must be heated above the melting point during erasing, whereas therecording layer is heated below the melting point during writing. Byensuring that the record carrier has a low reflection, hence a highabsorption, when the largest quantity of radiation is to be absorbed,i.e., during erasing, the power to be supplied by the radiation sourcemay remain limited.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter inaccordance with the drawings, in which:

FIG. 1 diagrammatically shows an information storage system;

FIG. 2 shows a cross-sectional view of a record carrier;

FIG. 3a shows an initially amorphous recording layer with writtencrystalline areas;

FIG. 3b shows a graphic representation of temperature profiles in arecording layer as a function of the location during writing and erasingin an initially amorphous recording layer;

FIG. 3c shows a recording layer erased by means of a known method;

FIG. 3d graphically shows temperature variation in a recording layer asa function of time during erasing; and

FIG. 4 graphically shows a write pulse and erase pulses for directoverwriting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an information storage system having (a) a record carrier 1and (b) a device 2 for writing information in the record carrier and forreading and erasing that information. In the device, a radiation beam 3is generated by a radiation source 4, preferably a diode laser. Anobjective system 5 focuses the radiation beam to a scanning spot 6 on arecording layer in the record carrier. At least a part of the radiationreflected by the record carrier is directed towards a detection system 8by a beam splitter 7, for example, a partially transparent mirror. Thedetection system generates a read signal S_(i) ' which represents theinformation which is stored on the record carrier from the reflectedradiation. The device also includes a pulse generator 9 which generatespulses in dependence upon signals at its inputs 10, 11 and 12. Aninformation signal S_(i) in which the information to be recorded iscoded may be applied to the input 10. A mode signal S_(m) whichindicates whether the pulse generator 9 must supply (a) a constantsignal at its output 13 for reading the record carrier, (b) write pulsesin accordance with the information signal S_(i), (c) erase pulses forerasing the information or (d) a combination thereof is applied to theinput 11. The read signal S_(i) ' may be applied to the input 12 so asto make the pulse generator 9 function in dependence upon theinformation stored on the record carrier. The device 2 still furtherincludes a modulator 14 which modulates the intensity of the radiationbeam with the signal(s) supplied by the pulse generator. As is shown bymeans of broken lines in FIG. 1, the output 13 of the pulse generator 9may be connected to the modulator 14 such that it is arranged in thepath of the radiation beam 3. Since the radiation beam of a diode lasercan be satisfactorily modulated by means of the current through thediode laser, the pulse generator 9 can also directly control theradiation source (if it is a diode laser for example) as isdiagrammatically shown in FIG. 1. In that case, the modulator isintegrated with the radiation source (i.e., a radiation source which canbe modulated).

The record carrier 1, a portion of which is shown in a cross-section inFIG. 2, comprises a transparent substrate 15 on which a stack of thefollowing thin layers are provided: a semi-transparent metal layer 16, adielectric layer 17, a recording layer 18, a subsequent dielectric layer19, a subsequent metal layer 10 and possibly a protective layer 21. Inthe embodiment of the record carrier shown in FIGS. 1 and 2, theradiation beam 3 is incident through the substrate 15 on the stack oflayers. The recording layer 18 consists of a phase-change material whichcan be switched by means of heating it by a radiation beam between astate with an amorphous structure and a state with a crystallinestructure. The information can be recorded in the recording layer 18 ina series of information areas, with the state of one structure in anenvironment with the state of the other structure. The information areasare preferably ordered in juxtaposed tracks. The scanning spot 6 scans atrack by moving the scanning spot 6 with respect to the recordcarrier 1. The information areas can be read by making use of thedifference in reflection between the two states of the phase-changematerial. The scanning beam reflected by the recording layer then has anintensity which is modulated by the succession of information areas. Theintensity modulation is converted in the detection system 8 into anelectric read signal S_(i) ', i.e., the read signal S_(i) ', which canbe processed in a known manner.

FIG. 3a shows a part of the recording layer 18 with two crystallineinformation areas 23 and 24 in an amorphous environment 28. Thecenterlines 25, 26 and 27 of three juxtaposed tracks are indicated bythe broken lines. The width of the tracks is equal, for example, to thewidth of the information areas, the tracks being separated byinformationless intermediate areas. When an information area is written,the recording layer is locally heated by the radiation beam from roomtemperature T_(c) to a temperature above the crystallization temperatureT_(r) and just below the melting point T_(m) of the recording layer 18.The temperature of the recording layer 18 as a function of the locationhas a bell-shaped profile 30 as is shown in FIG. 3b. As a result of thejust-mentioned heating, an area in the recording layer is with adiameter 31 (see FIG. 3b) is crystallized, i.e., the atoms in theamorphous structure order themselves in a crystalline structure. Theminimum time required for this ordering procedure to occur is referredto as the crystallization time. The power in the radiation beam, theperiod of time of the write pulse and the rate of cooling of therecording layer 18 (i.e., the dielectric layers 17 and 19) determine howlong the recording layer 18 will remain above a given temperature. Thecooling rate is determined by the thermal properties of the layers atboth sides of the recording layer 18 (i.e., the dielectric layers 17 and19). By correct choice of the pulse duration of the write pulse and thecooling rate, it is possible to have the temperature of the area to bewritten remain above the crystallization temperature long enough so asto cause the entire area to change from the amorphous state to thecrystalline state. In this way, information area 23 (as shown in FIG.3a) can be written. For areas having a larger length, such asinformation area 24 (see FIG. 3a), a write pulse having a longer pulseduration is used. This keeps a longer area above the crystallizationtemperature long enough for it to crystallize completely.

In order to erase the information areas, the crystalline structure ofthose areas must be converted back into an amorphous structure. To dothis, the recording layer 18 is heated above the melting point T_(m) byan erase pulse so that the recording layer 18 melts and subsequentlycools so rapidly at the end of the erase pulse that the recording area18 solidifies in the amorphous state. The temperature profile of therecording layer 18 at the end of the erase pulse is indicated by theprofile 32 in FIG. 3b. An area having a diameter 33 is melted by theerase pulse. The diameter 33 is preferably slightly larger than thediameter 31 of the information area in order to melt the completeinformation area. The rate at which the layer cools down to thecrystallization temperature after the end of the erase pulse may begiven a high value by manufacturing thin dielectric layers 17 and 19 ofa satisfactorily thermally conducting material and by giving the metallayer 20 a thickness which can rapidly dissipate the heat by conductionin the plane of the metal layer. If a continuous radiation beam wereused for erasing a track, the relatively long period of time duringwhich the beam remains above each point of the track would cause therecording layer to be heated well above the crystallization temperaturefor a long time so that the track to be erased would crystallizecompletely. To avoid this, the recording layer is heated with erasepulses having a pulse duration which is shorter than that of the writepulses, as is known, inter alia from the above-mentioned article in theIBM Technical Disclosure Bulletin.

Since the publication of the above-mentioned article in the IBMTechnical Disclosure Bulletin, it has become known that pulsed erasingof crystalline areas in an amorphous environment does not lead to thedesired situation of a fully amorphous recording layer. For this reason,record carriers in which crystalline effects are written in an amorphouslayer have been predominantly used for write-once record carriers inwhich the erasing problem does not occur.

The Applicant has come to the conclusion, confirmed by experiments, thatthe above-mentioned method of pulsed erasing leads to the results shownin FIG. 3c. FIG. 3c shows a recording layer 18 in which a track has beenerased with a series of erase pulses, in which the consecutive heatedareas overlap each other. Instead of a uniform amorphous track, a seriesof amorphous areas 34 surrounded by narrow crystalline areas 35 isobtained. The size of the narrow crystalline areas 35 depends on theerasing conditions and properties of the record carrier. At a subsequentwrite action, a series of information areas is written over the seriesof narrow crystalline areas 35, at which action the narrow crystallineareas 35 may subsist completely or partly between or on the edges of thenewly written information areas. Consequently, when the newly writteninformation is being read, the read signal S_(i) ' exhibits so muchnoise caused by the narrow crystalline areas 35, that the read signalS_(i) ' is difficult to process.

The origin of the narrow crystalline areas 35 can be explained withreference to FIG. 3b. The temperature distribution in the recordinglayer 18 at the location of the scanning spot at the end of the erasepulse is represented by means of the curve 32 of FIG. 3b. The centralarea below the heating scanning spot 6 will be heated above the meltingpoint T_(m) over a diameter 33. A surrounding, annular area 35 with aninner diameter 33 and an outer diameter 36 is heated to a temperaturebetween the crystallization temperature T_(c) and the melting pointT_(m). The annular area 35 will remain in this temperature trajectoryduring a part of the pulse duration of the erase pulse and the coolingtime of the recording layer 18. In the known erasing methods, this timeis longer than the crystallization time. Consequently, the annular areawill be entirely or partly changed from the amorphous structure to thecrystalline structure. After the radiation beam has been switched off,the annular area cools down in this crystalline structure. However, thecentral area, which is in its molten state during the erase pulse,solidifies at the end of the erase pulse due to the high cooling rate inthe amorphous structure. After erasing, narrow crystalline areas 35 areleft on the recording layer 18 within an area having a width ofapproximately diameter 36 minus diameter 33.

The present invention utilizes the above-mentioned recognition of thecause of the poor erasibility of the recording layer 18 and provides thepossibility of erasing the recording layer 18 satisfactorily. Accordingto the invention, a relatively long crystallization time should becombined with a relatively short erase pulse and a rapid cooling so thatthe annular area 35 is above the crystallization temperature for a timewhich is shorter than the crystallization time. Within the duration ofthe short erase pulse, the annular area 35 will be heated above thecrystallization temperature T_(c), likewise as with the known, longerase pulses. However, before the atoms are able to order themselves inthis area in a crystalline structure, i.e., within the crystallizationtime, the erase pulse is switched off and the recording layer cools downto below the crystallization temperature T_(c). Consequently, theannular area 35 maintains its amorphous structure. By applying the erasepulses in such a rapid succession that the heated areas form anuninterrupted track, all information written in a track can be erasedcompletely.

Whereas for unwanted crystallization of the annular areas 35 theduration of the erase pulse and the cooling time are important duringerasing, only the cooling time is important for rendering the centralarea 34 (with diameter 33) amorphous. The time spent in the molten stateis not important. Consequently, the designer of the record carrier willhave the freedom to adapt the cooling rate to the requirements forrendering the central area 34 amorphous by the choice of the layersaround the recording layer 18, while the crystallization time can beadapted by choosing the parameters of the recording layer 18 to theminimum pulse duration of the erase pulses to be realized by means ofthe device 2.

In addition to the crystallization time, the heating time of therecording layer is important for the pulse duration of the erase pulses.FIG. 3d shows the temperature variation T of the recording layer 18 as afunction of time t during erasing. This variation holds true for thepart of the annular area 35 heated to the highest temperature, i.e., thepart adjoining the inner diameter 33. The pulse duration of the erasepulse is denoted by t_(p). Before the erase pulse, the recording layer18 is at room temperature T_(r). At the end of the erase pulse, the partof the recording layer adjoining the inner diameter 33 of the annulararea 35 is heated to just below the melting point. The temperaturevariation during the erase pulse is shown as a straight line, which is areasonable approximation for heating with a short pulse. At the end ofthe erase pulse, the recording layer 18 cools down to ambient (i.e.,room) temperature as shown in the curve of FIG. 3b. The temperature ofthe recording layer 18 will be higher than the crystallizationtemperature T_(c) during a time t₁ during the erase pulse and a time t₂from the start of the cooling period. According to the invention, thesum of these times (i.e., t₁ and t₂) should be smaller than thecrystallization time of the recording layer 18 so as to prevent therecording layer 18 from crystallizing. The time t₂ is the previouslydefined cooling time. For different recording materials it holds that(T_(m) -T_(r))≈2(T_(c) -T_(r)). For example, for many GeTeSb alloys, itholds that T_(m) ≈630° C. and T_(c) ≈300° C. It will be evident that forsuch materials t₁ ≈1/2t_(p), i.e., the inner portion of the annular area35 is above the crystallization temperature T_(c) during half the pulseduration.

A record carrier in which the scanning rate is 1.3 m/s, as isconventional in record carriers of the compact disc type, willhereinafter be considered in describing an example of an implementationof the invention. The record carrier of the example has the structureshown in FIG. 2 and is designed for a radiation wavelength of 780 nm.Specifically, that record carrier has the following structure: atransparent substrate 15 with refractive index n=1.52, a metal layer 16of 11 nm thick gold with n=0.33-i6.2, a dielectric layer 17 of 142 nmTa₂ O₅ with n=2.1, a recording layer 18 of 10 nm Ge₄₂ Te₄₂ Sb₁₆ withn=4.53-i1.17, a dielectric layer 19 of 10 nm Ta₂ O₅, a metal layer 20 ofmore than 30 nm thick gold and a protective layer 21. (If the metallayer 20 is more than 30 nm thick, the transmission of the layer for theradiation is negligibly small.) The reflection of the record carrier ofthe example when the recording layer 18 is in the amorphous state is71%, and 27% when the recording layer 18 is in the crystalline state.Information areas are written on the record carrier of the example bymeans of a write pulse having a minimum pulse duration of 770 ns and apower which is sufficient to heat the recording layer 18 just below themelting point T_(m). By taking a crystallization time of, for example,400 ns, the recording layer 18 has sufficient time within the writepulse to crystallize so that a crystalline information area is written.The crystallization time is dependent, inter alia, on the chemicalcomposition of the recording layer 18. For a layer of, for example,GeSb₂ Te₄, the crystallization time can be varied by adding Sb. Withoutadditional Sb, the crystallization time is 50 ns, and with the materialGeSb₄ Te₄, i.e., with additional Sb, the crystallization time is 1 μs.The cooling rate of the recording layer 18 can be varied by means of thethickness of the type of metal of the metal layer 20 and the thicknessand thermal conduction of the dielectric layer 19. For the givenstructure of the record carrier of the example and a thickness of 30 nmof the gold layer 20, the cooling rate is 20 K/ns, and 30 K/ns at athickness of 100 nm. If the temperature varies linearly with time duringthe time t₂, as is shown in FIG. 3d, and if T_(c) and T_(m) have thepreviously mentioned values of 630° C. and 300° C., respectively, thecooling rate for a gold layer of 30 nm thick is approximately equal to16 ns. By erasing with pulses of 30 ns, the period of time when theinner portion of the annular (amorphous) area 35 is above thecrystallization temperature T_(c) is approximately 45 ns (i.e., equal to1/2*30+30= 45), which is shorter than the crystallization time.Consequently, the inner portion of the annular area 35 will notcrystallize and remains amorphous. The portion surrounding the annulararea has a lower temperature at the end of the erase pulse than theinner portion, as is clearly shown in FIG. 3b, and will consequentlycool down more rapidly below the crystallization temperature. Theportion surrounding the annular area 35 remains at a temperature abovethe crystallization temperature T_(c) for a shorter time than the innerportion. Moreover, the crystallization time increases with a decreasingtemperature so that the surrounding portion of the annular area 35 willcertainly remain amorphous during erasing. By working with an erasingpulse repetition time between 300 and 700 ns, a continuous amorphoustrack is obtained.

Previously written information can be directly overwritten with newinformation by supplying a write pulse during the time when aninformation area must be written and by supplying a series of erasepulses between successive write pulses. FIG. 4, which indicates thepower P in the radiation beam as a function of time t, shows a writepulse 40 preceded and succeeded by a series of erasing pulses 41. In thecase of direct overwriting, the write pulse can be advantageouslystarted with a preheating pulse 42, i.e., a short pulse having a higherpower than the subsequent part of the write pulse. The preheating pulseensures that the recording layer is rapidly heated so that writingstarts earlier than in the case of a write pulse without a preheatingpulse. This leads to a shorter transition between an erased and awritten area and provides the possibility of increasing the informationdensity. With a satisfactory ratio between the power in the preheatingpulse and the subsequent part of the write pulse, the writteninformation area acquires a uniform width over the length of theinformation area. The latter can also be realized by means of a writepulse comprising a series of pulses.

The intermediate areas located just outside the tracks on the recordinglayer, i.e., outside the diameter 33 in FIG. 3c will be heated above thecrystallization temperature T_(c) during erasing, but only for a shorttime so that those intermediate areas do not change to the crystallinestate during an erasing action. However, the phase-change material has amemory so that crystallization may still occur after repeated erasing.This memory does not lead to problems around the centerline because, dueto the layer melting, the memory is erased during each writing action.The unwanted crystallization of the intermediate areas may be obviatedby erasing the recording layer between the tracks once after a givennumber of erasing actions (i.e., the area in which the scanning spot 6does not scan along the centerline of the tracks). Such a change of thepath of the scanning spot can be realized in a known manner by meanscomprising a tracking control loop 37 which influences, for example, theposition of the objective system 5, and an electric circuit 38 forchanging the polarity of a tracking error signal S_(t) which controlsthe control loop 37 and is generated in the detection system 8. Due tothe polarity change, the scanning spot 6 no longer tracks the centerlineof a track but a path midway between two neighboring centerlines.

It is also possible to slow down the unwanted crystallization. This canbe realized by ensuring that there is a minimum number ofcrystallization nuclei between the tracks, so that there is a maximumcrystallization time. This can be realized by taking a recording layermaterial having fewer crystallization nuclei. To be able to write such alayer sufficiently rapidly, the number of crystallization nuclei aroundthe centerline of a track should be increased. To this end, a narrowgroove may be provided on the centerline, which groove has a width of,for example, one third of the track period and a V-shaped cross-section.It has been found that the sharp bottom and the edges of the groovefunction as crystallization nuclei.

The unwanted crystallization of the intermediate areas during erasing,caused by the memory of the recording layer, will occur earlier when thepulse duration of the erase pulse is of the same order as thecrystallization time. To prevent areas which are still in the amorphousstate from becoming crystalline in such a case, it is recommended thatthe erase pulses be generated only if the scanning spot is present incrystalline areas which must be erased. The erase pulse should not begenerated if the scanning spot is present in the areas which are alreadyamorphous. In the period between successive erase pulses, it is possibleto read the record carrier. Hence, in accordance with this erasingstrategy, erase pulses are generated or not generated dependent on thestate, either crystalline or amorphous, of the area where the scanningspot is present. An advantage of this erasing strategy is that the laseris subjected to a smaller load. For the purpose of reading, a radiationbeam having a low intensity must be incident on the recording layer 18between the erase pulses. If this method of reading-before-erasing isnot used, it is recommended that the beam be switched off completelybetween the pulses, for this will increase the cooling rate of therecording layer 18 at the end of an erase pulse.

As will be evident from the foregoing, the power absorbed by therecording layer 18 during erasing should be higher than during writingbecause the recording layer 18 must be heated to a higher temperatureduring erasing than during writing. In accordance with further aspectsof the invention, the maximum power to be supplied by the device 2 canbe limited by forming the record carrier 1 by means of suitable choiceof the layers 16, 17, 18, 19 and 20 such that its reflection with therecording layer 18 in the amorphous state is larger than that with therecording layer 18 in the crystalline state. If the record carrier has asmall transmission, the absorption in the crystalline state will then behigher, and a larger part of the incident power, than in the amorphousstate, will be absorbed. The power of the erase pulses is optimallyutilized by maximizing the absorption of the radiation in the recordinglayer 18 in the state in which the highest power in the recording layer18 is required, i.e. during erasing. It is true that the power of thewrite pulses is utilized less optimally, but this is not a drawbackbecause less power is required for writing. With a satisfactory choiceof the reflection of the record carrier in the two states, it ispossible to write and erase with write and erase pulses of the samepower. The pulse generator 9 of the device 2 should then be able togenerate only pulses of different pulse duration and equal value. Thismethod is quite suitable for directly overwriting information in asystem in which the possible positions of the information areas in therecording layer 18 are fixed. During overwriting, it can then be avoidedthat the scanning spot is located at an interface between an amorphousarea and a crystalline area during a pulse, so that the reflection,averaged across the scanning spot, would have an undefined value and thepulse would not yield the desired write or erasing result.

A record carrier which initially, in the amorphous state, has a highreflection, is compatible with record carriers of the compact disc typein the case of a suitable choice of the amorphous and crystallinereflections. The values of the reflections can be satisfactorilyrealized in record carriers having a stack of layers which, viewed fromthe radiation entrance side, comprise MIP, PIM, DIPIM or MIPIM, M is ametal layer, I is a dielectric interference layer, P is a recordinglayer of phase-change material and D is one or more dielectric layerstogether forming a reflector.

We claim:
 1. A record carrier having a recording layer containing atrack, the recording layer having more crystallization nuclei proximateto a centerline of the track than away from the centerline.
 2. Therecord carrier as claimed in claim 1, wherein the track has a groove inthe centerline thereof, and the groove has sharp interfaces in across-sectional direction of the track constituting crystallizationnuclei.
 3. A record carrier:(a) having a recording layer in whichinformation is writable therein by a write pulse radiation beam whichchanges an area of the recording layer which has an amorphous structureinto an information area having a crystalline structure; and (b) whichis scannable by an erase pulse radiation beam which erases aninformation area of the recording layer having a crystalline structureby changing the information area into an area having an amorphousstructure; wherein an area of the recording layer having an amorphousstructure has a reflection which is larger than an information area ofthe recording layer having a crystalline structure, and the write pulseradiation beam and the erase pulse radiation beam have amplitudes whichare substantially equal.